With reference to FIG. 1, a Rzeppa constant velocity universal joint serving as a fixed type constant velocity universal joint mainly includes an outer joint member 2, an inner joint member 3, balls 4, and a cage 5. In a spherical radially inner surface 8 of the outer joint member 2, a plurality of curved track grooves 6 are formed equiangularly along an axial direction. In a spherical radially outer surface 9 of the inner joint member 3, a plurality of curved track grooves 7 opposed to the track grooves 6 of the outer joint member 2 are formed equiangularly along the axial direction. A plurality of balls 4 configured to transmit torque are arranged between the track grooves 6 of the outer joint member 2 and the track grooves 7 of the inner joint member 3, respectively. The cage 5 configured to hold the balls 4 is arranged between the spherical radially inner surface 8 of the outer joint member 2 and the spherical radially outer surface 9 of the inner joint member 3. An outer periphery of the outer joint member 2 and an outer periphery of a shaft 12 coupled to the inner joint member 3 are covered with a boot 13, and grease serving as a lubricant is sealed inside the joint.
Curvature centers of the spherical radially inner surface 8 of the outer joint member 2 and the spherical radially outer surface 9 of the inner joint member 3 are each formed at a joint center O. In contrast, a curvature center A of each of the track grooves 6 of the outer joint member 2 and a curvature center B of each of the track grooves 7 of the inner joint member 3 are offset from the joint center O by an equal distance f1 to opposite sides in the axial direction. With this, when the joint forms an operating angle, the balls 4 are always guided on a plane bisecting an angle formed by axis lines of the outer joint member 2 and the inner joint member 3, and hence rotation torque is transmitted between the two axes at constant velocity.
A fixed type constant velocity universal joint 1 is an eight-ball Rzeppa constant velocity universal joint, and, as compared to related-art six-ball constant velocity universal joints, has a smaller track offset amount, a larger number of balls, and a smaller diameter, to thereby achieve a constant velocity universal joint, which secures strength, load capacity, and durability equal to or higher than those of fixed type universal joints using six balls and is lightweight, compact, and highly efficient with reduced torque loss. In order to reduce heat generation due to a large normal operating angle or during high speed rotation in such an eight-ball fixed type constant velocity universal joint, there has been proposed a fixed type constant velocity universal joint with focus on a pocket gap of a cage and on a spherical surface gap between an inner joint member and the cage (Patent Document 1).